The present invention generally pertains to fission chamber detector systems for monitoring neutron flux in a nuclear reactor, and is particularly directed to increasing the monitored range, improving alignment of processed signals derived from different portions of the monitored range, and providing high neutron signal sensitivity in a hostile environment.
A fission chamber detector is a type of neutron detector that is preferred for use in neutron flux monitoring systems because it has been proven to have a longer life and to be more reliable than other types of neutron detectors.
In a typical prior art fission chamber detector system for monitoring neutron flux in a nuclear reactor, a number of fission chambers are located inside a biological shield that surrounds the reactor core. Neutron signals produced in response to the detection of neutrons are transferred over conductors, such as coaxial cables to a preamplifier unit located inside a containment vessel for the reactor. The preamplifier unit amplifies the neutron signals for enabling further transmission via coaxial cables.
In prior art systems, the preamplifier units are located within the containment vessel for the nuclear reactor because in such prior art systems, the quality of the neutron signals would be so much diminished by electrical noise, attenuation, and signal reflection if the preamplifier units were located more than one hundred feet (thirty meters) from the neutron chambers that the sensitivity of the system would be impaired. The location of the preamplifier units within the containment vessel makes the preamplifier units susceptible to being rendered inoperable in the event that they are subjected to a hostile environment such as exists when the reactor suffers a loss of coolant accident. In the event of such an accident, the environment within the containment vessel is severely changed. Steam, boric acid, caustic sprays and other contaminants that are adverse to electrical circuits permeate the air, and the temperature and the air pressure increase to such an extent that preamplifier units in conventional containers would not withstand the increased temperature and would be damaged by such contaminants as penetrated the container under the conditions of increased pressure. Also the radiation level would increase to make the preamplifier units inoperative from the radiation damage. Yet, it is particularly important that neutron flux within the biological shield be monitored during and following a loss-of-coolant accident. This would require preamplifier units located within the containment vessel to be shielded from high radiation and temperature and to have containers that can withstand high pressure and be impermeable to contaminants. It is impractical and very expensive to meet this requirement.
It is desirable to monitor neutron flux over an extra wide range of up to twelve decades. However, most prior art fission chamber detector systems for monitoring neutron flux have a useful range of only ten decades. A decade is the range from 10.sup.n to 10.sup.n+1. In some prior art systems for monitoring neutron flux, the range has been extended to twelve decades by using proportional counters in combination with fission chamber detectors. However, proportional counters have a relatively short lifetime and their performance is rapidly degraded by the presence of gamma rays and the high temperatures surrounding the reactor core.
In processing the amplified signals to provide indications of reactor power and the rate of change of reactor power, the system utilizes pulse counting for the lower portion of the monitored range and a mean square voltage processing technique for the upper portion of the monitored range. In prior art systems difficulties have been encountered in aligning the pulse count signals with the mean square voltage signals. Heretofore, it has not been possible to obtain an accurate alignment with a simple calibration system, and it has been necessary to use a nuclear reactor in adjusting the alignment.
A particularly difficult problem in signal alignment that arises in prior art monitoring systems, such as described in U.S. Pat. No. 3,579,127 to Thomas, is the presence of spurious transients in processed signals indicating the rate of change of reactor power. These transients are caused during the transitions between the pulse count signals and the mean square voltage signals. To minimize this problem, prior art monitoring systems make use of complex bias and alignment circuitry and require expensive time consuming alignment procedures.